Is there any way to copy the weight of one bone over to another(forgot to use x-mirror)?
I'll show you the method that works for me.


Activate the 'corrective shapekeys' addon (file--> user preferences--> addons--> corrective shapekeys). It can be reached from the 'object data' menu (under the 'vertex groups' header) by clicking the black 'down arrow'.





In 'weight paint mode' select the bone with the assigned weight and click 'copy vertex group' in the corective shapekeys addon menu.





Now you have a thigh.L and the thigh.L_copy in 'vertex groups'. With the original one selected, click on the padlock icon and lock it. Next select the copy and click the 'mirror vertex groups (topology)' option.





Rename the thigh.L_copy to thigh.R. 





Now clicking the bone on the left and then the bone on the right, you can see that both have individual weights assigned to them.





The last thing is to assign the proper vertex groups to thigh.R bone. Select thigh.R bone and enter the 'edit mode' (TAB). Press A key to deselect everything and then press 'select' under 'vertex groups'. As you can see the right bone has assigned vertices that belong to the left one. We have to to remove it.





Unselect (B + mouse scroll) the vertices from the right side.





Remove the vertices from the left side by pressing 'remove' under 'vertex groups'.





Press A to deselect all and pres 'select' again. Now only the right side vertices are assigned to the right bone. 




The left side weights and assigned vertices have been transfered to the right side.